1. Field of the Invention
The present invention relates to a molten metal supply system and, more particularly, a continuous pressure molten metal supply system and method of operating the same.
2. Description of the Prior Art
The metal working process known as extrusion involves pressing metal stock (ingot or billet) through a die opening having a predetermined configuration in order to form a shape having a longer length and a substantially constant cross-section. For example, in the extrusion of aluminum alloys, the aluminum stock is preheated to the proper extrusion temperature. The aluminum stock is then placed into a heated cylinder. The cylinder utilized in the extrusion process has a die opening at one end of the desired shape and a reciprocal piston or ram having approximately the same cross-sectional dimensions as the bore of the cylinder. This piston or ram moves against the aluminum stock to compress the aluminum stock. The opening in the die is the path of least resistance for the aluminum stock under pressure. The aluminum stock deforms and flows through the die opening to produce an extruded product having the same cross-sectional shape as the die opening.
Referring to FIG. 1, the foregoing described extrusion process is identified by reference numeral 10, and typically consists of several discreet and discontinuous operations including: melting 20, casting 30, homogenizing 40, optionally sawing 50, reheating 60, and finally, extrusion 70. The aluminum stock is cast at an elevated temperature and typically cooled to room temperature. Because the aluminum stock is cast, there is a certain amount of inhomogeneity in the structure and the aluminum stock is heated to homogenize the cast metal. Following the homogenization step, the aluminum stock is cooled to room temperature. After cooling, the homogenized aluminum stock is reheated in a furnace to an elevated temperature called the preheat temperature. Those skilled in the art will appreciate that the preheat temperature is generally the same for each billet that is to be extruded in a series of billets and is based on experience. After the aluminum stock has reached the preheat temperature, it is ready to be placed in an extrusion press and extruded.
All of the foregoing steps relate to practices that are well known to those skilled in the art of casting and extruding. Each of the foregoing steps is related to metallurgical control of the metal to be extruded. These steps are very cost intensive, with energy costs incurring each time the metal stock is reheated from room temperature. There are also in-process recovery costs associated with the need to trim the metal stock, labor costs associated with process inventory, and capital and operational costs for the extrusion equipment.
Attempts have been made in the prior art to design an extrusion apparatus that will operate directly with molten metal. U.S. Pat. No. 3,328,994 to Lindemann discloses one such example. The Lindemann patent discloses an apparatus for extruding metal through an extrusion nozzle to form a solid rod. The apparatus includes a container for containing a supply of molten metal and an extrusion die (i.e., extrusion nozzle) located at the outlet of the container. A conduit leads from a bottom opening of the container to the extrusion nozzle. A heated chamber is located in the conduit leading from the bottom opening of the container to the extrusion nozzle and is used to heat the molten metal passing to the extrusion nozzle. A cooling chamber surrounds the extrusion nozzle to cool and solidify the molten metal as it passes therethrough. The container is pressurized to force the molten metal contained in the container through the outlet conduit, heated chamber and ultimately, the extrusion nozzle.
U.S. Pat. No. 4,075,881 to Kreidler discloses a method and device for making rods, tubes, and profiled articles directly from molten metal by extrusion through use of a forming tool and die. The molten metal is charged into a receiving compartment of the device in successive batches that are cooled so as to be transformed into a thermal-plastic condition. The successive batches build up layer-by-layer to form a bar or other similar article.
U.S. Pat. Nos. 4,774,997 and 4,718,476, both to Eibe, disclose an apparatus and method for continuous extrusion casting of molten metal. In the apparatus disclosed by the Eibe patents, molten metal is contained in a pressure vessel that may be pressurized with air or an inert gas such as argon. When the pressure vessel is pressurized, the molten metal contained therein is forced through an extrusion die assembly. The extrusion die assembly includes a mold that is in fluid communication with a downstream sizing die. Spray nozzles are positioned to spray water on the outside of the mold to cool and solidify the molten metal passing therethrough. The cooled and solidified metal is then forced through the sizing die. Upon exiting the sizing die, the extruded metal in the form of a metal strip is passed between a pair of pinch rolls and further cooled before being wound on a coiler.
An object of the present invention is to provide a molten metal supply system that may be used to supply molten metal to downstream metal working or forming processes at substantially constant working pressures. It is a further object of the present invention to provide a molten metal supply system incorporating a plurality of molten metal injectors adapted to generate relatively high working pressures with correspondingly low amounts of stored energy, and further exhibit improved wear resistance.
The foregoing objects are accomplished with a molten metal supply system and method of operating the same in accordance with the present invention. The molten metal supply system includes a molten metal supply source, a plurality of molten metal injectors, and a gas supply source. The plurality of molten metal injectors each include an injector housing and a piston reciprocally operable within the housing. The injector housing is configured to contain molten metal and is in fluid communication with the molten metal supply source. The piston is movable through a return stroke allowing molten metal to be received into the housing from the molten metal supply source, and a displacement stroke for displacing the molten metal from the housing to a downstream process. The piston has a pistonhead for displacing the molten metal from the housing. The gas supply source is in fluid communication with the housing of each of the injectors through respective gas control valves. During the return stroke of the piston of each of the injectors, a space is formed between the pistonhead and the molten metal and the corresponding gas control valve is operable to fill the space with gas from the gas supply source. During the displacement stroke of the piston of each of the injectors, the corresponding gas control valve is operable to prevent venting of gas from the gas filled space, such that the gas in the gas filled space is compressed between the pistonhead and the molten metal received into the housing and displaces the molten metal from the housing ahead of the piston.
The molten metal supply system may further include a control unit connected to each of the injectors and configured to individually actuate the injectors, such that the pistons move substantially serially through the return and displacement strokes thereby providing a substantially constant molten metal flow and pressure to the downstream process. The control unit may be configured to control the injectors such that at least one of the pistons moves through its displacement stroke while the remaining pistons move through their return strokes to provide the substantially constant molten metal flow and pressure to the downstream process. The piston of each of the injectors may include a piston rod having a first end and a second end. The first end may be connected to the pistonhead and the second end may be connected to an actuator for driving the piston through the return and the displacement strokes. The control unit may be connected to the actuator and the gas control valve of each of the injectors for controlling the operation of the actuator and the gas control valve.
An annular pressure seal may be positioned about the piston rod of each of the injectors, and provide a substantially gas tight seal between the piston rod and the housing of each of the injectors. A cooling water jacket may be positioned about the housing for each of the injectors and be located substantially coincident with the pressure seal for cooling pressure seal. The first end of the piston rod of each of the injectors may be connected to the pistonhead by a thermal insulation barrier. The piston rod of each of the injectors may define a central bore, with the central bore in fluid communication with a cooling water inlet and outlet for supplying cooling water to the central bore.
The molten metal supply source may contain aluminum, magnesium, bronze, iron, and alloys thereof. The gas supply source may include helium, nitrogen, argon, compressed air, and carbon dioxide.
A floating thermal insulation barrier may be located between the pistonhead and the molten metal received into the housing of each of the injectors. Each of the injectors may further include an intake/injection port connected to the housing for injecting the molten metal displaced from the housing to the downstream process. An outlet manifold may be in fluid communication with the intake/injection port of each of the injectors to receive molten metal displaced from the injectors. A check valve may be located in the intake/injection port of each of the injectors. The molten metal supply source may be in fluid communication with the housing of each of the injectors through the check valve located in the intake/injection port. A second check valve may be located in the intake/injection port of each of the injectors and be configured to allow the displacement of molten metal from the housing of each of the injectors to the output manifold.
The molten metal supply system may be configured to use a liquid medium as a viscous liquid source and pressurizing medium. The molten metal supply system, according to this second embodiment of the present invention, includes a molten metal supply source, a plurality of molten metal injectors, and a liquid chamber. The plurality of molten metal injectors each include an injector housing and a piston. The injector housing is configured to contain molten metal and is in fluid communication with the molten metal supply source. The piston is reciprocally operable within the housing. The piston is movable through a return stroke allowing molten metal to be received into the housing from the molten metal supply source, and a displacement stroke for displacing the molten metal from the housing to a downstream process. The piston has a pistonhead for displacing the molten metal from the housing. The liquid chamber is positioned above and in fluid communication with the housing of each of the injectors. The liquid chamber contains a liquid chemically resistive to the molten metal contained in the molten metal supply source. The liquid chamber is in fluid communication with the housing of each of the injectors such that during the return and displacement strokes of the piston within the housing liquid from the liquid chamber is located about the pistonhead and between the molten metal received into the housing and the liquid chamber.
The molten metal supply source may contain molten aluminum or aluminum alloy and the liquid in the liquid chamber may comprise boron oxide. The liquid chamber may be positioned directly on top of the housings of the injectors and the piston of each of the injectors may be reciprocally operable, such that during the return stroke of the piston, the pistonhead retracts at least partially upward into the liquid chamber.
The present invention is also a method of operating a molten metal supply system to supply molten metal to a downstream process at substantially constant molten metal flow rates and pressures. The method may comprise of steps of providing a molten metal supply system as generally described hereinabove; serially actuating the injectors to move the pistons through their return and displacement strokes at different times thereby providing a substantially constant molten metal flow rate and pressure to the downstream process; forming a space between the pistonhead and molten metal received into the housing during each respective return stroke of the pistons; filling the space with gas from the gas supply source during each respective return stroke of the pistons; and compressing the gas in the gas filled space formed between the pistonhead and the molten metal received into the housing of each of the injectors during each respective downstroke of the pistons to displace the molten metal from the housings of the injectors in advance of the compressed gas in the gas filled space. At least one of the pistons may be moving through its displacement stroke while the remaining pistons are moving through their return strokes to provide the substantially constant molten metal flow and pressure to the downstream process.
The method may include the step of venting the compressed gas in the gas filled space to atmospheric pressure approximately when the pistons respectively reach the end of their displacement strokes. The method may further include the steps of: respectively moving the pistons through a partial return stroke in their respective housings after the step of compressing the gas in the gas filled space to partially relieve the pressure in the compressed gas filled space; respectively venting the gas in the gas filled space to atmospheric pressure when the pistons are respectively located at the end of the partial return stroke in the housing; and respectively returning the pistons substantially to the end of their displacement strokes in the housings.
When the molten metal supply system is configured to operate with a liquid medium rather than a gas medium, the method may include the steps of: serially actuating the injectors to move the pistons through their return and displacement strokes at different times thereby providing substantially constant molten metal flow rate and pressure to the downstream process; and supplying liquid from the liquid chamber around the pistonhead and between the molten metal received into the housing and the liquid chamber of each of the injectors during the respective return and displacement strokes of the pistons. At least one of the pistons is preferably configured to move through its displacement stroke while the remaining pistons move through their return strokes to provide the substantially constant molten metal flow rate and pressure to the downstream process.
Further details and advantages of the present invention will become apparent from the following detailed description read in conjunction with the drawings, wherein like parts are designated with like reference numerals.